Tree encroachment dynamics in heathlands of north-west Italy: the fire regime hypothesis

Abstract

Tree encroachment is one of the primary conservation issues in Calluna-heathlands, a priority habitat in Europe. Improving understanding of the ecological factors that trigger transitions to woodlands is key to developing strategies for heathlands management. The irrational use of fire has been recognized as one of the key factors that drives the loss of heathlands of north-west Italy. The effect of high frequency pastoral burning on the replacement of heathlands by grasslands has been documented by several studies. The relationship between fire and tree encroachment is less clear. The paper examines the effect of the fire regime on the encroachment of Populus tremula L. and Betula pendula Roth. in the heathland. The study was carried out at the Managed Nature Reserve of Vauda (7° 41’ E, 45° 13’ N), which includes one of the most valuable heathlands of north-western Italy. The experimental design consisted of analysing the age structure, dendrometric variables and the species composition of three aspen and birch stands, circular-shaped and isolated within the heathland matrix. From 1986 to 2009 all stands experienced the same fire regime due to pastoral burning. Wildfires of similar behaviour occurred in 1998, 2003 and 2008 and determined the stand structure observed in 2009. The results evidenced that fire acts as a catalyst not only for seedlings establishment, as previously documented, but also for woodland expansion in the heathland. After initial establishment, stands showed a concentric encroachment dynamic, mainly due to aspen root suckering after post-fire stem mortality, whose steps of expansion coincided with the return interval of wildfires. Moreover, aspen determined the loss of heathland characteristic species, whose relative abundances were inversely correlated to aspen density along a gradient from the stands centre to the surrounding heathland. The regulation of current burning practices by prescribed burning, integrated with rational grazing, presents the next research questions to be addressed

Prescribed burning in Italy: issues, advances and challenges

“…In the 19 th century, the shortage of timber resources, forests privatization, and the development of timber industry (Puettmann et al 2009) required increased fire control and the prohibition of agro-pastoral use of fire (Pyne 1982). Similar dynamics occurred north and south of the Alps, so that in the last three centuries the use of fire has been increasingly limited and confined to the mere management of marginal agricultural and pastoral areas (Ascoli & Bovio 2010a, Conedera & Krebs 2010.…”

“…However, shepherds throughout the Alps still practice it occasionally (Conedera & Krebs 2010). This habit is controversial: on one hand, burning helps maintaining the pastures, on the other it is now applied surreptitiously in high fire danger seasons resulting in unclear effects on the ecosystem and ultimately higher costs for fire control systems (Ascoli & Bovio 2010a). These issues are common in central and southern Italy (Delogu & Cabiddu 2009), where the fire is widely used to remove stubble and regenerate pastures (Fig.…”

“…Today, livestock density reached a minimum, and fire is being used during the period of maximum danger (i.e., in late winter), with a consequent increase in area burned (Borghesio 2009). This fire regime is in part a consequence of fire bans and the loss of traditional knowledge (Ascoli & Bovio 2010a). Shepherds ignite fires surreptitiously, when they are sure that the fire will self-sustain and spread, so as to be able to leave the scene.…”

“…These uncontrolled fires burn under severe fire weather and spread over areas far greater than those necessary for pasture management. The combination of frequent fire and infrequent grazing causes the encroachment of fire-resilient tree species (e.g., aspen Populus tremula L.) at the expense of the heather (Ascoli & Bovio 2010a).…”

Prescribed burning experiences in Italy: an integrated approach to prevent forest fires

“…Un'ampia bibliografia documenta il ruolo ecologico del fuoco per conservare la brughiera, e dimostra come la tecnica del fuoco prescritto sia una soluzione gestionale efficace ed economica adottata in molte aree geografiche d'Europa per regolamentare l'uso rurale del fuoco (Lázaro 2010). Diversamente, alcuni studi in Piemonte hanno evidenziato come l'uso irrazionale del fuoco pastorale determini incendi troppo frequenti, intensi ed estesi che causano l'espansione del Populus tremula L. a scapito della brughiera e problemi nelle zone di interfaccia urbano-foresta (Borghesio 2009, Ascoli & Bovio 2010b). In collaborazione con l'Ente di Gestione della RNO e con il coinvolgimento di alcuni pastori che operano nell'area sono stati studiati trattamenti sperimentali di fuoco prescritto, integrati con il pascolo e lo sfalcio, con l'obiettivo di individuare idonee soluzioni gestionali ).…”